1. Field of the Invention
The present invention relates to the generation of laser light and more specifically to the generation of tunable laser light in the ultraviolet region of the electromagnetic spectrum.
2. Description of the Prior Art
Lasers can be categorized by the regions of the electromagnetic spectrum in which they operate. These regions include the infrared (IR), visible (Vis.), and ultraviolet (UV). Each region can be further divided into regions such as the near infrared (NIR) and vacuum ultraviolet (VUV). The greatest number of laser light sources are found in the IR and visible regions of the spectrum while fewer options are available in the UV and VUV regions.
The UV region is important because photons of these wavelengths contain enough energy to break chemical bonds and because short wavelengths can be focused more precisely than longer wavelengths. UV light sources are used in applications such as spectroscopy, optical testing, medicine, machining, and lithography. For example, ArF and KrF excimer lasers are frequently used for lithography in the semiconductor industry. The short wavelengths of these lasers' outputs enable high resolution in the resulting image. Unfortunately, these lasers have significant disadvantages including the use of toxic gasses, poor beam quality, poor power stability, and relatively broad linewidths. In large-scale applications their optical components can also be very expensive. There is, therefore, a great need for alternative light sources and calibration standards at UV wavelengths.
Common UV sources include excimer lasers and systems that rely upon the harmonic conversion of light from sources in the visible or IR regions. Excimers include ArF, KrF, and F2 gas lasers that generate light at approximately 193, 248, and 157 nm respectively. Also available is the N2 gas laser with an output near 337 nm.
Harmonic generation provides an alternative to direct generation of ultraviolet light. In this approach light is produced in the visible or IR regions and then converted to shorter wavelengths using non-linear optics such as birefringent crystals or gases. The shorter wavelengths are exact harmonics or differences between input wavelengths. Harmonic generation requires relatively high power input sources to produce higher harmonics because harmonic generation is a relatively inefficient non-linear process. Traditional input sources for harmonic generation systems include Nd:YAG, Nd:YLF, IR diode, CO2, and dye lasers.
UV sources that employ harmonic generation are generally limited by the original light sources whose fundamental outputs are typically at wavelengths above 1 micron. For example Neodymium lasers (1.064, 1.047, and 1.0535 μm) produce light at fixed wavelengths and cannot be tuned to produce harmonics at desirable 248 and 193 nm outputs. Tunable systems such as dye lasers and Alexandrite lasers also have disadvantages. Dye lasers, for example, are less powerful, more complex, and more difficult to operate than direct UV sources. Alexandrite based laser systems can provide outputs between 700 and 818 nm. However, these system are severely limited with respect to their maximum power output and repetition rates.